Rechargeable lithium battery

ABSTRACT

A rechargeable lithium battery includes a positive electrode including a positive current collector and a positive active material layer disposed on the positive current collector; and a negative electrode including a negative current collector, a negative active material layer disposed on the negative current collector, and a negative electrode functional layer disposed on the negative active material layer, wherein the positive active material layer includes a first positive active material including at least one of a composite oxide of metal selected from cobalt, manganese, nickel, and a combination thereof and lithium and a second positive active material including at least one of compounds represented by Chemical Formula 1 to Chemical Formula 4, and the negative electrode functional layer includes flake-shaped polyethylene particles and
 
Li x2 Mn 1-y2 M′ y2 A 2   [Chemical Formula 1]
 
Li x2 Mn 1-y2 M′ y O 2-z2 X z2   [Chemical Formula 2]
 
Li x2 Mn 2 O 4-z2 X z2   [Chemical Formula 3]
 
Li x2 Mn 2-y2 M′ y2 M″ z2 A 4   [Chemical Formula 4]
         wherein, 0.9≤x2≤1.1, 0≤y2≤0.5, 0≤z2≤0.5, M′ and M″ are the same or different and are selected from Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, As, Zr, Mn, Cr, Fe, Sr, V, and a rare earth element, and   wherein A is selected from O, F, S, and P and X is selected from F, S, and P.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2019-0052573, filed on May 3, 2019, which is herebyincorporated by reference for all purposes as if fully set forth herein.Further, two related co-pending applications were filed on Jul. 2, 2019with United States Patent and Trademark Office, as U.S. patentapplication Ser. No. 16/460,765 and U.S. patent application Ser. No.16/460,779, both of which are hereby incorporated by reference for allpurposes as if fully set forth herein, but are not admitted to be priorart with respect to the present invention by their mention in thecross-reference section.

BACKGROUND OF THE INVENTION Field

Exemplary embodiments/implementations of the invention relate generallyto rechargeable lithium battery.

Discussion of the Background

A portable information device such as a cell phone, a laptop, smartphone, and the like or an electric vehicle has used a rechargeablelithium battery having high energy density and easy portability as adriving power source. In addition, research on use of a rechargeablelithium battery as a power source for a hybrid or electric vehicle or apower storage by using high energy density characteristics has recentlybeen actively made.

One of the main research tasks of such a rechargeable lithium battery isto improve the safety of the rechargeable battery. For example, if therechargeable lithium battery exothermic due to internal short circuit,overcharge and over-discharge, and the like, an electrolytedecomposition and thermal runaway may occur, and internal pressureinside the battery may rapidly rise causing battery explosion. Amongthese, when internal short circuit of the rechargeable lithium batteryoccurs, there is a high risk of explosion because high electrical energystored in each electrode is conducted through the shorted positiveelectrode and negative electrode.

Explosion may cause fatal damages to the user. Therefore, it isimportant to develop a technology to improve the rechargeable lithiumbattery safety.

On the other hand, lithium iron phosphate (LFP) is used as a lowheat-generating safety material, but an average potential thereof isrelatively low, accompanied by a decrease in capacity when discharging.Therefore, there is a need for technology development to improve theseproblems.

The above information disclosed in this Background section is only forunderstanding of the background of the inventive concepts, and,therefore, it may contain information that does not constitute priorart.

SUMMARY

Devices constructed/methods according to exemplary implementationsembodiments of the invention are rechargeable lithium battery capable ofproviding high capacity and high power.

According to one or more implementations/embodiments of the invention, arechargeable lithium battery includes a positive electrode including apositive current collector and a positive active material layer disposedon the positive current collector; and a negative electrode including anegative current collector, a negative active material layer disposed onthe negative current collector, and a negative electrode functionallayer disposed on the negative active material layer, the positiveactive material layer includes a first positive active materialincluding at least one of a composite oxide of metal selected fromcobalt, manganese, nickel, and a combination thereof and lithium and asecond positive active material including at least one of compoundsrepresented by Chemical Formula 1 to Chemical Formula 4, and thenegative electrode functional layer includes flake-shaped polyethyleneparticles.Li_(x2)Mn_(1-y2)M′_(y2)A₂  [Chemical Formula 1]Li_(x2)Mn_(1-y2)M′_(y2)O_(2-z2)X_(z2)  [Chemical Formula 2]Li_(x2)Mn₂O_(4-z2)X_(z2)  [Chemical Formula 3]Li_(x2)Mn_(2-y2)M′_(y2)M″_(z2)A₄  [Chemical Formula 4]

-   -   wherein 0.9≤x2≤1.1, 0≤y2≤0.5, 0≤z2≤0.5, M′ and M″ are the same        or different and are selected from Mg, Al, Co, K, Na, Ca, Si,        Ti, Sn, V, Ge, Ga, B, As, Zr, Mn, Cr, Fe, Sr, V, and a rare        earth element, and    -   A is selected from O, F, S, and P and X is selected from F, S,        and P.

As the reaction rate depending on a temperature is improved, an earlyshut-down function may be implemented, thereby enabling high capacityand high power characteristics of the rechargeable lithium battery.

Additional features of the inventive concepts will be set forth in thedescription which follows, and in part will be apparent from thedescription, or may be learned by practice of the inventive concepts.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate exemplary embodiments of theinvention, and together with the description serve to explain theinventive concepts.

FIG. 1 schematically shows a structure of a rechargeable lithium batteryaccording to an embodiment of the present disclosure.

FIG. 2 is a graph showing discharge rate characteristics of therechargeable battery cells according to Example 1 and comparativeexample.

FIG. 3 is a SEM photograph of polyethylene particles of a negativeelectrode functional layer according to an embodiment.

FIG. 4 is a SEM photograph of a negative electrode composition accordingto an embodiment.

DETAILED DESCRIPTION

In the following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of various exemplary embodiments or implementations of theinvention. As used herein “embodiments” and “implementations” areinterchangeable words that are non-limiting examples of devices ormethods employing one or more of the inventive concepts disclosedherein. It is apparent, however, that various exemplary embodiments maybe practiced without these specific details or with one or moreequivalent arrangements. In other instances, well-known structures anddevices are shown in block diagram form in order to avoid unnecessarilyobscuring various exemplary embodiments. Further, various exemplaryembodiments may be different, but do not have to be exclusive. Forexample, specific shapes, configurations, and characteristics of anexemplary embodiment may be used or implemented in another exemplaryembodiment without departing from the inventive concepts.

Unless otherwise specified, the illustrated exemplary embodiments are tobe understood as providing exemplary features of varying detail of someways in which the inventive concepts may be implemented in practice.Therefore, unless otherwise specified, the features, components,modules, layers, films, panels, regions, and/or aspects, etc.(hereinafter individually or collectively referred to as “elements”), ofthe various embodiments may be otherwise combined, separated,interchanged, and/or rearranged without departing from the inventiveconcepts.

The use of cross-hatching and/or shading in the accompanying drawings isgenerally provided to clarify boundaries between adjacent elements. Assuch, neither the presence nor the absence of cross-hatching or shadingconveys or indicates any preference or requirement for particularmaterials, material properties, dimensions, proportions, commonalitiesbetween illustrated elements, and/or any other characteristic,attribute, property, etc., of the elements, unless specified. Further,in the accompanying drawings, the size and relative sizes of elementsmay be exaggerated for clarity and/or descriptive purposes. When anexemplary embodiment may be implemented differently, a specific processorder may be performed differently from the described order. Forexample, two consecutively described processes may be performedsubstantially at the same time or performed in an order opposite to thedescribed order. Also, like reference numerals denote like elements.

When an element, such as a layer, is referred to as being “on,”“connected to,” or “coupled to” another element or layer, it may bedirectly on, connected to, or coupled to the other element or layer orintervening elements or layers may be present. When, however, an elementor layer is referred to as being “directly on,” “directly connected to,”or “directly coupled to” another element or layer, there are nointervening elements or layers present. To this end, the term“connected” may refer to physical, electrical, and/or fluid connection,with or without intervening elements. Further, the D1-axis, the D2-axis,and the D3-axis are not limited to three axes of a rectangularcoordinate system, such as the x, y, and z axes, and may be interpretedin a broader sense. For example, the D1-axis, the D2-axis, and theD3-axis may be perpendicular to one another, or may represent differentdirections that are not perpendicular to one another. For the purposesof this disclosure, “at least one of X, Y, and Z” and “at least oneselected from the group consisting of X, Y, and Z” may be construed as Xonly, Y only, Z only, or any combination of two or more of X, Y, and Z,such as, for instance, XYZ, XYY, YZ, and ZZ. As used herein, the term“and/or” includes any and all combinations of one or more of theassociated listed items.

Although the terms “first,” “second,” etc. may be used herein todescribe various types of elements, these elements should not be limitedby these terms. These terms are used to distinguish one element fromanother element. Thus, a first element discussed below could be termed asecond element without departing from the teachings of the disclosure.

Spatially relative terms, such as “beneath,” “below,” “under,” “lower,”“above,” “upper,” “over,” “higher,” “side” (e.g., as in “sidewall”), andthe like, may be used herein for descriptive purposes, and, thereby, todescribe one elements relationship to another element(s) as illustratedin the drawings. Spatially relative terms are intended to encompassdifferent orientations of an apparatus in use, operation, and/ormanufacture in addition to the orientation depicted in the drawings. Forexample, if the apparatus in the drawings is turned over, elementsdescribed as “below” or “beneath” other elements or features would thenbe oriented “above” the other elements or features. Thus, the exemplaryterm “below” can encompass both an orientation of above and below.Furthermore, the apparatus may be otherwise oriented (e.g., rotated 90degrees or at other orientations), and, as such, the spatially relativedescriptors used herein interpreted accordingly.

The terminology used herein is for the purpose of describing particularembodiments and is not intended to be limiting. As used herein, thesingular forms, “a,” “an,” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. Moreover,the terms “comprises,” “comprising,” “includes,” and/or “including,”when used in this specification, specify the presence of statedfeatures, integers, steps, operations, elements, components, and/orgroups thereof, but do not preclude the presence or addition of one ormore other features, integers, steps, operations, elements, components,and/or groups thereof. It is also noted that, as used herein, the terms“substantially,” “about,” and other similar terms, are used as terms ofapproximation and not as terms of degree, and, as such, are utilized toaccount for inherent deviations in measured, calculated, and/or providedvalues that would be recognized by one of ordinary skill in the art.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure is a part. Terms,such as those defined in commonly used dictionaries, should beinterpreted as having a meaning that is consistent with their meaning inthe context of the relevant art and should not be interpreted in anidealized or overly formal sense, unless expressly so defined herein.

Hereinafter, as an example of a rechargeable lithium battery, acylindrical rechargeable lithium battery is for example described. FIG.1 schematically shows a structure of a rechargeable lithium batteryaccording to an embodiment. Referring to FIG. 1 , a rechargeable lithiumbattery 100 according to an embodiment includes a battery cell includinga positive electrode 114, a negative electrode 112 facing the positiveelectrode 114, a separator 113 disposed between the positive electrode114 and the negative electrode 112, and an electrolyte (not shown)impregnating the positive electrode 114, negative electrode 112, andseparator 113, a battery case 120 containing the battery cell, and asealing member 140 sealing the battery case 120.

Hereinafter, a detailed configuration of the rechargeable lithiumbattery 100 according to an embodiment of the present invention isdescribed.

A rechargeable lithium battery according to an embodiment includes apositive electrode and a negative electrode. The positive electrodeincludes a positive current collector and a positive active materiallayer disposed on the positive current collector. The negative electrodeincludes a negative current collector, a negative active material layerdisposed on the negative current collector, and a negative electrodefunctional layer disposed on the negative active material layer.

The positive active material layer may include a first positive activematerial including at least one of a composite oxide of metal selectedfrom cobalt, manganese, nickel, and a combination thereof and lithium.It may also include a second positive active material including at leastone of compounds represented by Chemical Formula 1 to Chemical Formula4. The negative electrode functional layer includes flake-shapedpolyethylene particles.

The first positive active material may include a high capacity materialhaving a high energy density per unit content, thereby realizing highcapacity of the rechargeable lithium battery.

In addition, the second positive active material is a high powermaterial having a high average potential and thus may improve capacitydegradation at the high voltage discharge.

In other words, the first and second positive active materials are usedtogether to realize a high-capacity and high power rechargeable lithiumbattery.

In addition, the negative electrode including the negative electrodefunctional layer including the flake-shaped polyethylene particles, maymore effectively shut down ion channel and thus prevent an additionalelectrical/chemical reaction and thereby improve a rechargeable lithiumbattery safety.

The second positive active material has an average potential in a rangeof about 3.5 to 4.5 V. With average potential within such range, arechargeable lithium battery may work at a high voltage, improving thecapacity degradation problem, even at the increased discharge voltage,which may greatly improve cycle-life characteristics.

The first positive active material may be included in an amount of about80 wt % to about 99 wt % and specifically, about 85 wt % to about 99 wt%, for example, about 85 wt % to about 97 wt % based on a total weightof the positive active material layer.

The second positive active material may be included in an amount ofabout 1 wt % to about 20 wt % and specifically, about 1 wt % to about 15wt %, for example, about 3 wt % to about 15 wt % based on the totalweight of the positive active material layer.

In a specific embodiment, the first and second positive active materialsmay be included in a weight ratio of about 85:15 to about 99:1 or about85:15 to about 97:3, for example, about 85:15 to about 95:5, or about85:15 to about 90:10.

When the first and second positive active materials satisfy the ranges,the safety may be improved without the capacity degradation.

In addition, the second positive active material is a low exothermicmaterial and thus may lower an initial heat-increasing rate and securesafety.

The positive active material layer may further include a positiveelectrode functional layer disposed on the positive active materiallayer.

For example, the first positive active material may be included in thepositive active material layer, and the second positive active materialmay be included in the positive electrode functional layer.

In this case, the first positive active material and the second positiveactive material may be included in a weight ratio of about 85:15 toabout 99:1, for example, about 85:15 to about 97:3, or about 90:10 toabout 97:3.

For example, the first positive active material may be included in thepositive active material layer, and the second positive active materialmay be included in the positive active material layer and the positiveelectrode functional layer, respectively.

In this case, the first positive active material and the second positiveactive material may be included in a weight ratio of about 85:15 toabout 99:1, for example, about 89:11 to about 99:1.

In this case, the second positive active material of the positiveelectrode functional layer may be included in an amount of about 20parts by weight to about 120 parts by weight based on 100 parts byweight of the second positive active material of the positive activematerial layer.

The first positive active material may include one of LiCoO₂, Li_(x1)M¹_(1-y1-z1)M² _(y1)M³ _(Z1)O₂ (0.9≤x1≤1.8, 0≤y1≤1, 0≤z1≤1, 0≤y1+z1≤1, andM¹, M², and M³ are independently a metal of Ni, Co, Mn, Al, Sr, Mg, orLa), and a combination thereof.

For example, the first positive active material may include LiCoO₂, butis not limited thereto.

For example, M¹ may be Ni, and M² and M³ may independently be a metalsuch as Co, Mn, Al, Sr, Mg, or La.

More specifically, M¹ may be Ni, M² may be Co, and M³ may be Mn or Al,but is are not limited thereto.

The second positive active material may be at least one of compoundsrepresented by Chemical Formula 1 to Chemical Formula 4.Li_(x2)Mn_(1-y2)M′_(y2)A₂  [Chemical Formula 1]Li_(x2)Mn_(1-y2)M′_(y)O_(2-z2)X_(z2)  [Chemical Formula 2]Li_(x2)Mn₂O_(4-z2)X_(z2)  [Chemical Formula 3]Li_(x2)Mn_(2-y2)M′_(y2)M″_(z2)A₄  [Chemical Formula 4]

-   -   wherein, 0.9≤x2≤1.1, 0≤y2≤0.5, 0≤z2≤0.5, M′ and M″ are the same        or different and are selected from Mg, Al, Co, K, Na, Ca, Si,        Ti, Sn, V, Ge, Ga, B, As, Zr, Mn, Cr, Fe, Sr, V, and a rare        earth element, and    -   wherein A is selected from O, F, S, and P and X is selected from        F, S, and P.

For example, the second positive active material may be LiMn₂O₄, but isnot limited thereto.

The positive active material layer may optionally further include apositive electrode conductive material and a positive electrode binder.

The amounts of the positive electrode conductive material and thepositive electrode binder may be about 1 wt % to about 5 wt % based on atotal weight of the positive active material layer, respectively.

The positive electrode conductive material is used to impartconductivity to the positive electrode, and may be used as long as it isan electron conductive material without causing chemical change in thebattery. Examples of the conductive material may include a carbon-basedmaterial such as natural graphite, artificial graphite, carbon black,acetylene black, ketjen black, a carbon fiber, and the like; ametal-based material of a metal powder or a metal fiber includingcopper, nickel, aluminum, silver, and the like; a conductive polymersuch as a polyphenylene derivative; or a mixture thereof.

The positive electrode binder adheres positively to the positive activematerial particles, and also serves to adhere the positive activematerials to the current collector well. Examples thereof may bepolyvinyl alcohol, carboxylmethyl cellulose, hydroxypropyl cellulose,diacetyl cellulose, polyvinylchloride, carboxylated polyvinylchloride,polyvinylfluoride, an ethylene oxide-containing polymer,polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene,polyvinylidene fluoride, polyethylene, polypropylene, astyrene-butadiene rubber, an acrylated styrene-butadiene rubber, anepoxy resin, nylon, and the like, but are not limited thereto.

The positive current collector may include aluminum, nickel, and thelike, but is not limited thereto.

The electrolyte includes a non-aqueous organic solvent and a lithiumsalt.

The non-aqueous organic solvent serves as a medium for transporting ionstaking part in the electrochemical reaction of a battery.

The non-aqueous organic solvent may include a carbonate-based,ester-based, ether-based, ketone-based, alcohol-based, or aproticsolvent. The carbonate-based solvent may include dimethyl carbonate(DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropylcarbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate(MEC), ethylene carbonate (EC), propylene carbonate (PC), butylenecarbonate (BC), and the like and the ester-based solvent may includemethyl acetate, ethyl acetate, n-propyl acetate, dimethylacetate,methylpropionate, ethylpropionate, γ-butyrolactone, decanolide,valerolactone, mevalonolactone, caprolactone, and the like. Theether-based solvent may include dibutyl ether, tetraglyme, diglyme,dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, and the likeand the ketone-based solvent may include cyclohexanone, and the like.The alcohol-based solvent include ethyl alcohol, isopropyl alcohol, andso on, and examples of the aprotic solvent include nitriles such as R—CN(wherein R is a C2 to C20 linear, branched, or cyclic hydrocarbon groupthat may include a double bond, an aromatic ring, or an ether bond),amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane,sulfolanes, and so on.

The non-aqueous organic solvent may be used alone or in a mixture of twoor more. When the organic solvent is used in a mixture, the mixtureratio can be controlled in accordance with a desirable batteryperformance.

The carbonate-based solvent may include a mixture of a cyclic carbonateand a chain carbonate. The cyclic carbonate and the chain carbonate aremixed together at a volume ratio of about 1:1 through about 1:9, andwhen the mixture is used as an electrolyte, the electrolyte performancemay be enhanced.

The non-aqueous organic solvent of the present disclosure may furtherinclude an aromatic hydrocarbon-based organic solvent in addition to thecarbonate-based solvent. In this case, the carbonate-based solvent andthe aromatic hydrocarbon-based organic solvent may be mixed in a volumeratio of about 1:1 through about 30:1.

As the aromatic hydrocarbon-based organic solvent, an aromatichydrocarbon-based compound of Chemical Formula 5 may be used.

-   -   wherein R₁ to R₆ are the same or different and are selected from        hydrogen, a halogen, a C1 to C10 alkyl group, a haloalkyl group,        and a combination thereof.

Specific examples of the aromatic hydrocarbon-based organic solvent maybe selected from benzene, fluorobenzene, 1,2-difluorobenzene,1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene,1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene,1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene,1,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene,1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene,1,2,4-triiodobenzene, toluene, fluorotoluene, 2,3-difluorotoluene,2,4-difluorotoluene, 2,5-difluorotoluene, 2,3,4-trifluorotoluene,2,3,5-trifluorotoluene, chlorotoluene, 2,3-dichlorotoluene,2,4-dichlorotoluene, 2,5-dichlorotoluene, 2,3,4-trichlorotoluene,2,3,5-trichlorotoluene, iodotoluene, 2,3-diiodotoluene,2,4-diiodotoluene, 2,5-diiodotoluene, 2,3,4-triiodotoluene,2,3,5-triiodotoluene, xylene, and a combination thereof.

The non-aqueous electrolyte may further include vinylene carbonate or anethylene carbonate-based compound of Chemical Formula 6 in order toimprove cycle-life of a battery.

-   -   wherein R₇ and R₈ may be the same or different and may be        selected from hydrogen, a halogen group, a cyano group (CN), a        nitro group (NO₂), and a fluorinated C1 to C5 alkyl group,        wherein at least one of R₇ and R₈ is selected from a halogen        group, a cyano group (CN), a nitro group (NO₂), and a        fluorinated C1 to C5 alkyl group, provided that R₇ and R₈ are        not both hydrogen.

Examples of the ethylene carbonate-based compound may include difluoroethylenecarbonate, chloroethylene carbonate, dichloroethylene carbonate,bromoethylene carbonate, dibromoethylene carbonate, nitroethylenecarbonate, cyanoethylene carbonate, or fluoroethylene carbonate. Theamount of the cycle-life improvement additive may be used within anappropriate range.

The lithium salt dissolved in an organic solvent supplies a battery withlithium ions, basically operates the rechargeable lithium battery, andimproves transportation of the lithium ions between a positive electrodeand a negative electrode. Examples of the lithium salt include at leastone supporting salt selected from LiPF₆, LiBF₄, LiSbF₆, LiAsF₆,LiN(SO₂C₂F₅)₂, Li(CF₃SO₂)₂N, LiN(SO₃C₂F₅)₂, LiC₄F₉SO₃, LiClO₄, LiAlO₂,LiAlCl₄, LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂) wherein, x and y arenatural numbers, LiCl, LiI, and LiB(C₂O₄)₂ (lithium bis(oxalato) borate,LiBOB). A concentration of the lithium salt may range from about 0.1 Mto about 2.0 M. When the lithium salt is included at the aboveconcentration range, an electrolyte may have excellent performance andlithium ion mobility due to optimal electrolyte conductivity andviscosity.

The polyethylene is generally HDPE (high density polyethylene, density:about 0.94 g/cc to about 0.965 g/cc), MDPE (medium density polyethylene,density: about 0.925 g/cc to about 0.94 g/cc), LDPE (low densitypolyethylene, density: about 0.91 g/cc to about 0.925 g/cc), VLDPE (verylow density polyethylene, density: about 0.85 g/cc to about 0.91 g/cc),and the like.

The flake-shaped polyethylene particles may be used alone or incombination of two or more polyethylene polymers such as HDPE, MDPE, orLDPE.

The average particle size (D50) of the flake-shaped polyethyleneparticles included in the negative electrode functional layer disposedon the negative active material layer may be about 1 μm to about 8 μm,and specifically about 2 μm to about 6 μm.

As used herein, when a definition is not otherwise provided, the averageparticle size (D50) may be measured by a well-known method for a personof an ordinary skill in the art, for example, as a particle sizeanalyzer, or from TEM or SEM photographs. Alternatively, a dynamiclight-scattering measurement device is used to perform a data analysis,and the number of particles is counted for each particle size range.From this, the (D50) value may be easily obtained through a calculation.

On the other hand, a ratio of the long axis length relative to the shortaxis length of the flake-shaped polyethylene particles may be about 1 toabout 5, specifically about 1.1 to about 4.5, for example about 1.2 toabout 3.5.

In addition, a thickness of the flake-shaped polyethylene particles maybe about 0.2 μm to about 4 μm, specifically, about 0.3 μm to about 2.5μm, for example may be about 0.3 μm to about 1.5 μm.

The polyethylene particles according to this disclosure areflake-shaped, as seen in FIG. 3 , and the average particle size may bedefined as (D50) described above.

When the size and thickness of the flake-shaped polyethylene particlesare within the above range, ion channels may be effectively closed evenin a small amount of the flake-shaped polyethylene.

When the negative electrode functional layer including the flake-shapedpolyethylene particles is provided, a reaction rate may be increasedaccording to temperature under the same reaction conditions, comparedwith the case of including spherical polyethylene particles, therebyimproving safety improvement effect of the rechargeable lithium battery.In the case of the flake-shaped polyethylene particles before melting,an area covering pores is thinner and wider than that of the sphericalshape polyethylene particles before melting. When the polyethyleneparticles are melted at a predetermined temperature or more to close ionchannels, a reaction rate is faster because the flake-shapedpolyethylene particles have a larger area than that of the electrodeplate closed by the melted spherical polyethylene particles.

That is, the polyethylene particles included in the negative electrodefunctional layer during thermal runaway of the battery is melted toclose the ion channel, thereby limiting the movement of the ions toimplement a shut-down function may prevent additional electrochemicalreactions.

For example, as shown in FIG. 4 , since the flake-shaped polyethyleneparticles according to the embodiment are disposed in a thin and wideshape on the pores in a composition for the negative electrodefunctional layer, the flake-shaped polyethylene particles melts morerapidly during thermal runaway due to thermal/physical impact, therebysuppressing passage of ions.

The negative electrode functional layer may further include inorganicparticles and a binder.

A sum amount of the flake-shaped polyethylene particles and theinorganic particles over an amount of the binder may be a weight ratioof about 80:20 through about 99:1, and specifically, a weight ratio ofabout 85:15 through about 97:3.

The flake-shaped polyethylene particles and the inorganic particles maybe included in a weight ratio of about 95:5 through about 10:90, andspecifically in a weight ratio of about 30:70 through about 70:30.

The amounts of the flake-shaped polyethylene particles and the inorganicparticles within the above range, may secure cycle-life characteristicsand output characteristics of a battery.

The inorganic particles may include, for example, Al₂O₃, SiO₂, TiO₂,SnO₂, CeO₂, MgO, NiO, CaO, GaO, ZnO, ZrO₂, Y₂O₃, SrTiO₃, BaTiO₃,Mg(OH)₂, boehmite, or a combination thereof, but are not limitedthereto. Organic particles such as an acrylic compound, an imidecompound, an amide compound, or a combination thereof may be furtherincluded in addition to the inorganic particles, but are not limitedthereto.

The inorganic particles may be spherical, flake-shaped, cubic, oramorphous. The inorganic particles may have an average particle diameterof about 1 nm to about 2500 nm, for example about 100 nm to about 2000nm, about 200 nm to about 1000 nm, or about 300 nm to about 800 nm. Theaverage particle diameter of the inorganic particle may be an averageparticle size (D₅₀) at a volume ratio of 50% in a cumulativesize-distribution curve.

The negative electrode functional layer may have a thickness of about 1μm to about 10 and specifically about 3 μm to about 10 μm.

In addition, a ratio of the thickness of the negative active materiallayer to the thickness of the negative electrode functional layer may beabout 50:1 through about 10:1, and specifically about 30:1 through about10:1.

The negative electrode functional layer with the thickness of withinabove-mentioned range, may significantly improve the thermal safetywhile maintaining excellent cycle-life characteristics.

In particular, when the ratio of the thickness of the negative electrodefunctional layer is included in the above range, thermal safety may beimproved while minimizing the decrease in energy density.

The negative current collector may include one selected from a copperfoil, a nickel foil, a stainless steel foil, a titanium foil, a nickelfoam, a copper foam, a polymer substrate coated with a conductive metal,and a combination thereof.

The negative active material may include a material that reversiblyintercalates/de-intercalates lithium ions, a lithium metal, a lithiummetal alloy, a material capable of doping/de-doping lithium, or atransition metal oxide.

Examples of the material capable of reversiblyintercalating/de-intercalating the lithium ions may include acarbonaceous material, that is, a carbon-based negative active materialgenerally used in a rechargeable lithium battery. Examples of thecarbon-based negative active material may be crystalline carbon,amorphous carbon, or a combination thereof. The crystalline carbon maybe graphite such as non-shaped, sheet-shaped, flake-shaped, sphericalshape, or fiber shaped natural graphite or artificial graphite, and theamorphous carbon may be a soft carbon, a hard carbon, a mesophase pitchcarbonization product, fired coke, and the like.

The lithium metal alloy includes an alloy of lithium and a metalselected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba,Ra, Ge, Al, and Sn.

The material capable of doping/de-doping lithium may be a silicon-basedor a tin-based material, for example, Si, SiO_(x) (0<x<2), a Si-Q alloy(wherein Q is an element selected from an alkali metal, analkaline-earth metal, a Group 13 element, a Group 14 element, a Group 15element, a Group 16 element, a transition metal, a rare earth element,and a combination thereof, but not Si), a Si-carbon composite, Sn, SnO₂,a Sn—R alloy (wherein R is an element selected from an alkali metal, analkaline-earth metal, a Group 13 element, a Group 14 element, a Group 15element, a Group 16 element, a transition metal, a rare earth element,and a combination thereof, but not Sn), a Sn-carbon composite and thelike. At least one of these materials may be mixed with SiO₂. Theelements Q and R may be selected from Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr,Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs,Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb,Bi, S, Se, Te, Po, and a combination thereof.

The transition metal oxide may include a lithium titanium oxide.

In the negative active material layer, an amount of the negative activematerial about 95 wt % to about 99 wt % based on a total weight of thenegative active material layer.

The negative active material layer may optionally further include anegative electrode conductive material and a negative electrode binder.

Each amount of the negative electrode conductive material and negativeelectrode binder may be about 1 wt % to about 5 wt % based on a totalweight of the negative active material layer.

The negative electrode conductive material is used to impartconductivity to the negative electrode, and types of the negativeelectrode conductive material is the same as types of the positiveelectrode conductive material described above.

The negative electrode binder improves binding properties of negativeactive material particles with one another and with a current collector.The negative electrode binder may be a non-water-soluble binder, awater-soluble binder, an amphiphilic binder (awater-soluble/non-water-soluble binder), or a combination thereof.

The non-water-soluble binder may be polyvinylchloride, carboxylatedpolyvinylchloride, polyvinylfluoride, an ethylene oxide-containingpolymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene,polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide,polyimide, or a combination thereof.

The water-soluble binder may be a styrene-butadiene rubber, an acrylatedstyrene-butadiene rubber, polyvinyl alcohol, sodium polyacrylate, acopolymer of propylene and a C2 to C8 olefin, a copolymer of(meth)acrylic acid and (meth)acrylic acid alkyl ester, or a combinationthereof.

The amphiphilic binder may be an acrylated styrene-based rubber.

When the water-soluble binder is used as a negative electrode binder, acellulose-based compound may be further used to provide viscosity as athickener. The cellulose-based compound includes one or more ofcarboxylmethyl cellulose, hydroxypropylmethyl cellulose, methylcellulose, or alkali metal salts thereof. The alkali metals may be Na,K, or Li. The thickener may be included in an amount of about 0.1 partsby weight to about 3 parts by weight based on 100 parts by weight of thenegative active material.

The rechargeable lithium battery according to an embodiment of thepresent invention have the negative electrode functional layer includingthe flake-shaped polyethylene particles on the negative electrode andsimultaneously, the positive active material layer including the lithiumcobalt oxide (LCO)-based active material and the lithium manganese oxide(LMO)-based active material and thus may minimize the capacitydegradation and simultaneously, lower the heat-increasing rate accordingto thermal/physical impacts and thus effectively exhibit a shut-downeffect.

On the other hand, the separator 113 may be disposed between thepositive electrode 111 and the negative electrode 112 as describedabove. The separator 113 may be, for example, selected from a glassfiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene,or a combination thereof. It may have a form of a non-woven fabric or awoven fabric. For example, in a rechargeable lithium battery, apolyolefin-based polymer separator such as polyethylene andpolypropylene is mainly used. In order to ensure the heat resistance ormechanical strength, a coated separator including a ceramic component ora polymer material may be used. Optionally, it may have a mono-layeredor multi-layered structure.

Hereinafter, the above aspects of the present disclosure are illustratedin more detail with reference to examples. However, these examples areexemplary, and the present disclosure is not limited thereto.

Manufacture of Rechargeable Lithium Battery Cells Example 1

96 wt % of a positive active material prepared by mixing LiCoO2/LiMn2O4in a weight ratio of 95:5 as first/second positive active materials, 3wt % of a polyvinylidene fluoride binder, and 1 wt % of a ketjen blackconductive material were mixed in an N-methylpyrrolidone solvent toprepare positive active material slurry. The positive active materialslurry was coated on both surfaces of an aluminum current collector andthen, dried and compressed to manufacture a positive electrode having apositive active material layer.

98 wt % of graphite, 0.8 wt % of carboxylmethyl cellulose, and 1.2 wt %of a styrene-butadiene rubber were mixed in pure water to preparenegative active material slurry. The negative active material slurry wascoated on both surfaces of a copper current collector and then, driedand compressed to manufacture a negative electrode having a negativeactive material layer.

48 wt % of flake-shaped 2 μm PE particles (a long axis length/a shortaxis length=about 2, a thickness=about 0.6 μm), 47 wt % of alumina (anaverage particle diameter (D50)=0.7 μm), and 5 wt % of an acylatedstyrene-based rubber binder were mixed in an alcohol-based solvent toprepare PE/alumina slurry.

The PE/alumina slurry was coated on both surface of the negativeelectrode and then, dried and compressed to manufacture a negativeelectrode having a coating layer and the flake-shaped PE particles.

The positive electrode, a separator formed of a PE/PP multi-layer, andthe negative electrode including the coating layer including theflake-shaped PE particles were sequentially stacked to form an electrodeassembly having a structure shown in FIG. 1 , and an electrolyte (1.0 MLiPF6 in EC/DEC=50:50 v/v) was injected thereinto to manufacture arechargeable battery cell.

Comparative Example and Examples 2 and 3

Rechargeable lithium battery cells were respectively manufacturedaccording to the same method as Example 1 except that the cells wererespectively manufactured to have structures shown in Table 1.

Example 4

LiCoO₂ as a first positive active material, polyvinylidene fluoride as abinder, and ketjen black as a conductive material in a weight ratio of96:3:1 were dispersed in an N-methylpyrrolidone solvent to preparepositive active material slurry. The positive active material slurry wascoated on both of the surface of an aluminum current collector and then,dried and compressed to form a positive active material layer.

In addition, in order to form a positive electrode functional layer,LiMn₂O₄ as a second positive active material and PVDF as a binder weredispersed in a weight ratio of 95:5 in an NMP solvent to preparepositive electrode functional layer slurry.

A rechargeable lithium battery cell was manufactured according to thesame method as Example 1 except that the positive electrode functionallayer slurry was loaded on the positive active material layer in anamount of 3 wt % based on a total weight of the positive active materialand then, coated, dried, and compressed to form a positive electrodefunctional layer and thus manufacture a positive electrode.

The positive electrode finally included 99 wt % of LiCoO₂ and 1 wt % ofLiMn₂O₄.

Examples 5 and 6

Rechargeable lithium battery cells were manufactured according to thesame method as Example 4 except that the cells were respectivelymanufactured to have structures shown in Table 2.

Example 7

96 wt % of a positive active material prepared by mixing LiCoO₂/LiMn₂O₄in a weight ratio of 94:5 as first/second positive active materials, 3wt % of a polyvinylidene fluoride binder, and 1 wt % of a ketjen blackconductive material were mixed in an N-methylpyrrolidone solvent toprepare positive active material slurry. The positive active materialslurry was coated on both surfaces of an aluminum current collector andthen, dried and compressed to manufacture a positive active materiallayer having a positive active material layer.

In addition, in order to form a positive electrode functional layer,LiMn₂O₄ as a second positive active material and PVDF as a binder weredispersed in a weight ratio of 95:5 in an NMP solvent to preparepositive electrode functional layer slurry.

A rechargeable lithium battery cell was manufactured according to thesame method as Example 1 except that the positive electrode functionallayer slurry was loaded in an amount of 1 wt % based on a total weightof the positive active material on the positive active material layerand then, coated, dried, and compressed to form a positive electrodefunctional layer and thus manufacture a positive electrode.

The positive electrode finally included 94 wt % of LiCoO₂ and 6 wt % ofLiMn₂O₄.

Examples 8 and 9

Rechargeable lithium battery cells were manufactured according to thesame method as Example 7 except that the cells were respectivelymanufactured to have structures shown in Table 3.

Evaluation Examples

1. Evaluation of Penetration Safety

The rechargeable lithium battery cells of Example 1 to 9 and comparativeexample were used to perform a penetration experiment at 5 mm/s by usinga pin having a diameter of 2.5 mm at a voltage of 4.3 V, and the resultsare shown in Tables 1 through 3.

<Evaluation Criteria>

-   -   L1: No influence on appearance    -   L2: Scratches on appearance but no leakage    -   L3: Leakage    -   L4: Fire    -   L5: Explosion

2. Evaluation of Dropping Safety

The rechargeable lithium battery cells according to Examples 1 through 9and comparative example were 18 times respectively dropped from 1.8 mhigh at 20° C.±5° C. When the battery cells were placed to have animpact on several places, the dropping test was three times performedwith respect to a top end/a bottom end/a right top angle/a right bottomangle/a left top angle/a left bottom angle, and the results are shown inTables 1 through 3.

3. Evaluation of Collision Safety

The rechargeable lithium battery cells according to Examples 1 through 9and comparative example were charged at 0.5 C under a cut-off of 0.05 Cup to a maximum voltage and aged for 24 hours and then, evaluated asfollows:

-   -   A full-charged cell was put on a steel plate (a thickness≥5 cm).    -   A round bar (15 mm) was placed in the center of a specimen,        wherein the round bar should be placed in a vertical direction        with an electrode.    -   A cylindrical cell was evaluated by placing the round bar in the        center like a seesaw shape.    -   A cylindrical weight of 9 kg was freely dropped from 610 mm high        to the bar, and the results were examined and shown in Tables 1        through 3.

TABLE 1 Positive electrode composition (Mixture) 1^(st) positive 2^(nd)positive active active material material Pene- Colli- (wt %) (wt %)tration Dropping sion Ex. 1 95 5 L4 L4 L4 Ex. 2 90 10 L2 L2 L2 Ex. 3 8515 L2 L2 L2 Comp. Ex. 100 0 L5 L4 L5

TABLE 2 Positive Positive active electrode material functional layerlayer 1^(st) positive 2^(nd)positive active active material materialPene- Colli- (wt %) (wt %) tration Dropping sion Ex. 4 99 1 L4 L4 L4 Ex.5 97 3 L2 L2 L2 Ex. 6 94 6 L2 L2 L2 Comp. Ex 100 0 L5 L4 L5

TABLE 3 Positive electrode Positive active material functional layer(Mixture) layer 2^(nd) positive 2^(nd) positive 1^(st) active activeactive material material material Pene- Drop- (wt %) (wt %) (wt %)tration ping Collision Ex. 7 94 5 1 L2 L2 L2 Ex. 8 92 5 3 L2 L2 L2 Ex. 989 5 6 L2 L2 L2 Ex. 1 95 5 0 L4 L4 L4

In Tables 1 through 3, in the penetration, dropping, and collisionsafety evaluations, comparative example including no second positiveactive material was ignited or exploded, but most of the samplescomprising the second positive active material according to variousExamples exhibited no leakage and thus relatively sufficientpenetration/dropping/collision safety characteristics.

FIG. 2 is a graph showing discharge rate characteristics of therechargeable battery cells according to Example 1 and comparativeexample.

Referring to Tables 1 through 3 and FIG. 2 , the rechargeable lithiumbattery cells of Examples simultaneously included the second positiveactive material and the first positive active material and exhibitedimproved safety without charge capacity degradation.

Although certain exemplary embodiments and implementations have beendescribed herein, other embodiments and modifications will be apparentfrom this description. Accordingly, the inventive concepts are notlimited to such embodiments, but rather to the broader scope of theappended claims and various obvious modifications and equivalentarrangements as would be apparent to a person of ordinary skill in theart.

What is claimed is:
 1. A rechargeable lithium battery, comprising: apositive electrode comprising: a positive current collector; and apositive active material layer disposed on the positive currentcollector; and a negative electrode comprising: a negative currentcollector; a negative active material layer disposed on the negativecurrent collector; and a negative electrode functional layer disposed onthe negative active material layer, wherein the negative electrodefunctional layer comprises: flake-shaped polyethylene particles, andwherein the positive active material layer comprises: a first positiveactive material including at least one of a composite oxide of metalselected from cobalt, manganese, nickel, and a combination thereof andlithium; and a second positive active material including at least one ofcompounds represented by Chemical Formula 1 through Chemical Formula 4,Li_(x2)Mn_(1-y2)M′_(y2)A₂  [Chemical Formula 1]Li_(x2)Mn_(1-y2)M′_(y2)O_(2-z2)X_(z2)  [Chemical Formula 2]Li_(x2)Mn₂O_(4-z2)X_(z2)  [Chemical Formula 3]Li_(x2)Mn_(2-y2)M′_(y2)M″_(z2)A₄  [Chemical Formula 4] wherein inChemical Formula 1 through Chemical Formula 4, 0.9≤x2≤1.1, 0≤y2≤0.5,0≤z2≤0.5, M′ and M″ are the same or different and are selected from Mg,Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, As, Zr, Mn, Cr, Fe, Sr, V,and a rare earth element, and A is selected from O, F, S, and P and X isselected from F, S, and P, wherein the positive active material layerfurther comprises a positive electrode functional layer disposed on thepositive active material layer, wherein the first positive activematerial is included in the positive active material layer, and whereinthe second positive active material is included in the positiveelectrode functional layer.
 2. The rechargeable lithium battery of claim1, wherein the second positive active material has an average potentialof about 3.5 V to about 4.5 V.
 3. The rechargeable lithium battery ofclaim 1, wherein the first positive active material is included in anamount of about 80 wt % to about 99 wt % based on a total weight of thepositive active material layer.
 4. The rechargeable lithium battery ofclaim 1, wherein the first positive active material and the secondpositive active material are included in a weight ratio of about 85:15to about 99:1.
 5. The rechargeable lithium battery of claim 1, whereinthe first positive active material and the second positive activematerial are included in a weight ratio of about 85:15 to about 99:1. 6.A rechargeable lithium battery comprising: a positive electrodecomprising: a positive current collector; and a positive active materiallayer disposed on the positive current collector; and a negativeelectrode comprising: a negative current collector; a negative activematerial layer disposed on the negative current collector; and anegative electrode functional layer disposed on the negative activematerial layer, wherein the negative electrode functional layercomprises: flake-shaped polyethylene particles, and wherein the positiveactive material layer comprises: a first positive active materialincluding at least one of a composite oxide of metal selected fromcobalt, manganese, nickel, and a combination thereof and lithium; and asecond positive active material including at least one of compoundsrepresented by Chemical Formula 1 through Chemical Formula 4,Li_(x2)Mn_(1-y2)M′_(y2)A₂  [Chemical Formula 1]Li_(x2)Mn_(1-y2)M′_(y2)O_(2-z2)X_(z2)  [Chemical Formula 2]Li_(x2)Mn₂O_(4-z2)X_(z2)  [Chemical Formula 3]Li_(x2)Mn_(2-y2)M′_(y2)M″_(z2)A₄  [Chemical Formula 4] wherein inChemical Formula 1 through Chemical Formula 4, 0.9≤x2≤1.1, 0≤y2≤0.5,0≤z2≤0.5, M′ and M″ are the same or different and are selected from Mg,Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, As, Zr, Mn, Cr, Fe, Sr, V,and a rare earth element, and A is selected from O, F, S, and P and X isselected from F, S, and P, wherein the first positive active material isincluded in the positive active material layer, and wherein the secondpositive active material is included in the positive active materiallayer and the positive electrode functional layer, respectively.
 7. Therechargeable lithium battery of claim 6, wherein the first positiveactive material and the second positive active material are included ina weight ratio of about 85:15 to about 99:1.
 8. The rechargeable lithiumbattery of claim 6, wherein the second positive active material of thepositive electrode functional layer is included in an amount of about 20parts by weight to about 120 parts by weight based on 100 parts byweight of the second positive active material of the positive activematerial layer.
 9. The rechargeable lithium battery of claim 1, whereinthe first positive active material comprises one of LiCoO₂, Li_(x1)M¹_(1-y1-z1)M² _(y1)M³ _(Z1)O₂ (0.9≤x1≤1.8, 0≤y1≤1, 0≤z1≤1, 0≤y1+z1≤1, andM¹, M², and M³ are independently a metal of Ni, Co, Mn, Al, Sr, Mg, orLa), and a combination thereof.
 10. The rechargeable lithium battery ofclaim 1, wherein the second positive active material comprises LiMn₂O₄.11. The rechargeable lithium battery of claim 1, wherein theflake-shaped polyethylene particles has an average particle size (D50)of about 1 μm to about 8 μm.
 12. The rechargeable lithium battery ofclaim 1, wherein a ratio of a long axis length relative to a short axislength of the flake-shaped polyethylene particles is about 1 to about 5.13. The rechargeable lithium battery of claim 1, wherein a thickness ofthe flake-shaped polyethylene particles is about 0.2 μm to about 4 μm.14. The rechargeable lithium battery of claim 1, wherein the negativeelectrode functional layer further comprises: inorganic particles; and abinder.
 15. The rechargeable lithium battery of claim 14, wherein a sumamount of the flake-shaped polyethylene particles and the inorganicparticles over an amount of the binder is a weight ratio of about 80:20to about 99:1.
 16. The rechargeable lithium battery of claim 14, whereinthe flake-shaped polyethylene particles and the inorganic particles areincluded in a weight ratio of about 95:5 to about 10:90.
 17. Therechargeable lithium battery of claim 1, wherein the negative electrodefunctional layer is about 1 μm to about 10 μm thick.